Setting Log Scale on just y-axis of a TH2F graph

ROOT
,
1

I am trying to make a 2-D histogram of kinetic energy and PROMPT photoelectrons, but trying to set a log scale on the y-axis. The y-axis changes to a log scale but the tick marks dont appear properly and someone told me that it is not centered at 0. I tried “correlation->GetYaxis()->SetRangeUser(1e-1, 5000);” but this did not work. An image of the graph is shown here.

image
image1398×759 10.2 KB

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    auto canvas = new TCanvas("Canvas","Canvas",1400,900);
    auto correlation = new TH2F("Kinetic-PromptPE","Kinetic-PromptPE",50,0,250,50,0,5000);
    for(size_t i = 0 ;i < promptPE.size(); i++){
        if(!kinetic[i].empty()){
            correlation->Fill(kinetic[i][kinetic[i].size()-1],promptPE[i]);
        }

    }
    canvas->Draw();
    correlation->Draw("COLZ");
    gStyle->SetOptStat(0);
    gPad->SetLogy(1);
    correlation->GetXaxis()->SetTitle("Kinetic Energy / 5 (MeV)");
    correlation->GetYaxis()->SetTitle("Prompt PE / 100");
    canvas->SaveAs("/exp/sbnd/app/users/ipatel/NC_trigger/plot/Kinetic-PromptPE.png","recreate");

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2

Try instead (before Drawing)

correlation->SetMinimum(1e-1);
3

So this only changes the minimum for the z-axis, shown in this image below. I need it on the y-axis alone, so it shows more tickmarks on the y-axis.

Screenshot 2024-11-04 at 12.43.01 PM
Screenshot 2024-11-04 at 12.43.01 PM2040×1102 207 KB

4

?? It changes the minimum of the y-axis. Which do you think are the x,y,z axes? x,y are the usual, and z is the colour scale on the right side.

5

I read somewhere that the SetMinimum for TH2F graph makes the z-axis(the color) minimum to 0. Also the other issue that I stated was that the tick-marks on the y-axis dont appear as they normally do for 0,10^2,then`10^3 and so on.

6

Hi,

Nice explanation at https://root.cern.ch/doc/v630/classTH1.html:

One can use TH1::SetMaximum() and TH1::SetMinimum() to force a particular value for the maximum or the minimum scale on the plot. (For 1-D histograms this means the y-axis, while for 2-D histograms these functions affect the z-axis).

What do mean by that? First of, 0 is impossible to show on a log-scale plot. Second, you have the 10^3 tick shown on your plot: ticks below correspond to 9*10^2, 8*10^2, 7*10^2, …, 2*10^2. The ticks above are for 2*10^3, 3*10^3, and 4*10^3. What exactly is wrong with this?

7

Indeed, sorry for the confusion. If what you want is to change the lowest exponent shown on the axis, try SetLimits:

 {
  auto c1    = new TCanvas("c1","c1",1500,500);
  c1->Divide(3,1);

  auto hcol1 = new TH2F("hcol1","Original",40,-4,4,40,0,700);
  float px, py;
  for (Int_t i = 0; i < 50000; i++) {
    gRandom->Rannor(px,py);
    hcol1->Fill(px,100*py);
  }

  c1->cd(1);
  hcol1->DrawCopy("COLZ");

  c1->cd(2);
  gPad->SetLogy(1);
  hcol1->SetTitle("Logy");
  hcol1->DrawCopy("COLZ");

  c1->cd(3);
  gPad->SetLogy(1);
  hcol1->SetTitle("Logy + SetLimits");
  hcol1->GetYaxis()->SetLimits(1e-1,500);
  hcol1->DrawCopy("COLZ");
}

c1
c11498×476 10.2 KB

Edit: Note, by the way, that SetLimits may not be the best option, as it can alter your histogram (see this).

8

My advisor just told me that the y-axis is not actually beginning at 0, I understand for a logarithmic scale you cant really have that, but somewhere close should do. Also she said that because the only labeled tickmarks that were showing were the 10^3 and none of the others. So, I was wondering how to actually set the bottom to 0 and get more labeled tickmarks.

9 
h1col->GetYaxis()->SetMoreLogLabels();
10

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